Detection of opposite polarities in a sunspot light bridge: evidence of low-altitude magnetic reconnection

A multiwavelength photometric analysis was performed in order to study the sub-structure of a sunspot light bridge in the photosphere and the chromosphere. Active region NOAA 8350 was observed on 1998 October 8. The data consist of a 100 min time series of 2D spectral scans of the lines Fe  i 5576 Å, Hα 6563 Å, Fe  i 6302.5 Å, and continuum images at 5571 Å. We recorded line-of-sight magnetograms in 6302.5 Å. The observations were taken at the Dunn Solar Telescope at US National Solar Observatory, Sacramento Peak. We find evidence for plasma ejection from a light bridge followed by Ellerman bombs. Magnetograms of the same region reveal opposite polarity in light bridge with respect to the umbra. These facts support the notion that low-altitude magnetic reconnection can result in the magnetic cancellation as observed in the photosphere.     

The solar dynamo in the light of the distribution of various sunspot magnetic classes over butterfly diagram

We present the data concerning the distribution of various sunspot magnetic classes over the solar butterfly diagram and discuss how this data can inform solar dynamo models. We use the statistics of sunspots that violate the Hale polarity law to estimate the ratio of the fluctuating and mean components of the toroidal magnetic field inside the solar convective zone. An analysis of the spatial distribution of bipolar, unipolar and complex sunspot groups in the context of simple dynamo models results in the conclusion that the mean toroidal field is relatively simple and maintains its shape during the course of the solar cycle (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Search for relationship between duration of the extended solar cycles and amplitude of sunspot cycle

Duration of the extended solar cycles is taken into the consideration. The beginning of cycles is counted from the moment of polarity reversal of large-scale magnetic field in high latitudes, occurring in the sunspot cycle n till the minimum of the cycle n + 2. The connection between cycle duration and its amplitude is established. Duration of the “latent” period of evolution of extended cycle between reversals and a minimum of the current sunspot cycle is entered. It is shown, that the latent period of cycles evolution is connected with the next sunspot cycle amplitude and can be used for the prognosis of a level and time of a sunspot maximum. The 24th activity cycle prognosis is made. The found dependences correspond to transport dynamo model of generation of solar cyclicity, it is possible with various speed of meridional circulation. Long-term behavior of extended cycle's lengths and connection with change of a climate of the Earth is considered. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Center to limb variation of penumbral Stokes V profiles

We investigated the horizontal and the vertical component of the Evershed flow (EF). To this end, we computed average Stokes V profiles for various velocity classes in penumbrae at different heliocentric angles. Our results show that for blueshifted profiles an additional lobe with the same polarity as the spot is present in the blue side of the average Stokes V profile. The amplitude of the additional lobe grows with increasing blueshift and with increasing heliocentric angle. For small redshifts, the profiles show an additional lobe with the opposite polarity as the spot on the red side of the average Stokes V profile. Even at disk center, the original polarity of the average Stokes V profile is reversed for strong redshifts. The transition between the different types of Stokes V profiles is continuous and indicates that not only the vertical, but also the horizontal EF is a magnetized stream of plasma in a magnetic background field (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Probing sunspot magnetic fields with p-mode absorption and phase shift data

Long-standing observations of incoming and outgoing f- and p-modes in annuli around sunspots reveal that the spots partially absorb and substantially shift the phase of waves incident upon them. The commonly favoured absorption mechanism is partial conversion to slow magneto-acoustic waves that disappear into the solar interior channelled by the magnetic field of the sunspot. However, up until now, only f-mode absorption could be accounted for quantitatively by this means. Based on vertical magnetic field models, the absorption of p-modes was insufficient. In this paper, we use the new calculations of Crouch & Cally for inclined fields, and a simplified model of the interaction between spot interior and exterior. We find excellent agreement with phase shift data assuming field angles from the vertical in excess of 30° and Alfvén/acoustic equipartition depths of around 600–800 km. The absorption of f-modes produced by such models is considerably larger than is observed, but consistent with numerical simulations. On the other hand, p-mode absorption is generally consistent with observed values, up to some moderate frequency dependent on radial order. Thereafter, it is too large, assuming absorbing regions comparable in size to the inferred phase-shifting region. The excess absorption produced by the models is in stark contrast with previous calculations based on a vertical magnetic field, and is probably due to finite mode lifetimes and excess emission in acoustic glories. The excellent agreement of phase shift predictions with observational data allows some degree of probing of subsurface field strengths, and opens up the possibility of more accurate inversions using improved models. Most importantly, though, we have confirmed that slow mode conversion is a viable, and indeed the likely, cause of the observed absorption and phase shifts.     

Magnetic flux participation in solar surface magnetism during solar cycle 24

This study aims at investigating surface magnetic flux participation among different types of magnetic features during solar cycle 24. State-of-the-art observations from SDO/HMI and Hinode/SOT are combined to form a unique database in the interval from April 2010 to October 2015. Unlike previous studies, the statistics presented in this paper are feature-detection-based. More than 20 million magnetic features with relatively large scale, such as sunspot/pore, enhanced and quiet networks, are automatically detected and categorized from HMI observations, and the internetwork features are identified from SOT/SP observations. The total flux from these magnetic features reaches 5.9×10²² Mx during solar minimum and2.4 × 10²³ Mx in solar maximum. Flux occupation from the sunspot/pore region is 29% in solar maximum.Enhanced and quiet networks contribute 18% and 21% flux during the solar minimum, and 50% and 9% flux in the solar maximum respectively. The internetwork field contributes over 55% of flux in the solar minimum, and its flux contribution exceeds that of sunspot/pore features in the solar maximum. During the solar active condition, the sunspot field increases its area but keeps constant flux density of about 150 G,while the enhanced network follows the sunspot number variation showing increasing flux density and area,but the quiet network displays decreasing area and somewhat increasing flux density of about 6%. The origin of the quiet network is not known exactly, but is suggestive of representing the interplay between mean-field and local dynamos. The source, magnitude and possible importance of ‘hidden flux' are discussed in some detail.     

Spectropolarimetery of umbral fine structures from Hinode: evidence for magnetoconvection

We present spectropolarimetric analysis of umbral dots and a light bridge fragment that show dark lanes in G -band images. Umbral dots show upflow as well as associated positive Stokes V area asymmetry in their central parts. Larger umbral dots show downflow patches in their surrounding parts that are associated with negative Stokes V area asymmetry. Umbral dots show weaker magnetic field in central part and higher magnetic field in peripheral area. Umbral fine structures are much better visible in total circularly polarized light than in continuum intensity. Umbral dots show a temperature deficit above dark lanes. The magnetic field inclination shows a cusp structure above umbral dots and a light bridge fragment. We compare our observational findings with 3D magnetohydrodynamic simulations.     

The hemispheric sign rule of current helicity during the rising phase of cycle 23

We compute the signs of two different current helicity parameters (i.e., α_best andH _c) for 87 active regions during the rise of cycle 23 The results indicate that 59% of the active regions in the northern hemisphere have negative α_best and 65% in the southern hemisphere have positive. This is consistent with that of the cycle 22. However, the helicity parameterH _cshows a weaker opposite hemispheric preference in the new solar cycle. Possible reasons are discussed.     

The association between sunspot magnetic fields and superpenumbral fibrils

Spectropolarimetric observations of a sunspot were carried out with the Tenerife Infrared Polarimeter at Observatorio del Teide, Tenerife, Spain. Maps of the physical parameters were obtained from an inversion of the Stokes profiles observed in the infrared Fe I line at 15648 Å The regular sunspot consisted of a light bridge which separated the two umbral cores of the same polarity. One of the arms of the light bridge formed an extension of a penumbral filament which comprised weak and highly inclined magnetic fields. In addition, the Stokes V profiles in this filament had an opposite sign as the sunspot and some resembled Stokes Q or U. This penumbral filament terminated abruptly into another at the edge of the sunspot, where the latter was relatively vertical by about 30°. Chromospheric Hα and He II 304 Å filtergrams revealed three superpenumbral fibrils on the limb‐side of the sunspot, in which one fibril extended into the sunspot and was oriented along the highly inclined penumbral counterpart of the light bridge. An intense, elongated brightening was observed along this fibril that was co‐spatial with the intersecting penumbral filaments in the photosphere. Our results suggest that the disruption in the sunspot magnetic field at the location of the light bridge could be the source of reconnection that led to the intense chromospheric brightening and facilitated the supply of cool material in maintaining the overlying superpenumbral fibrils. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of noise on sunspot magnetic field parameters

Using a perturbated (noised) dipole model of a sunspot magnetic field structure we simulated the influence of background noise or apparent noise (unresolved small-scale magnetic field structure) on sunspot magnetic field parameters. We evaluated mean values of the vertical and horizontal electric current densities |j|_∥ and |j|_⊥, respectively, of the force-free parameter α and of the Lorentz force |F|. For comparison we estimated |j_⊥| and |F| of a standard sunspot magnetic field model (return-flux model, OSHEROVICH 1982). Furthermore, we compared our results with those from observations resulting in estimated values of |j|_∥ for quiet sunspots. Our investigation led to the following results: the estimated values of 〈|F|〉 show clearly that due to the noise the axisymmetric magnetic dipole model is clustered into several subsystems of fluxbundles. The latter are connected with a system of electric current densities of the order of |j|_∥ ∼ 10⁻³ Am⁻² and |j|_⊥ = 10⁻¹ Am⁻², i.e., this system is a noise-generated nonaxisymmetric magnetohydrostatic model.     

Horizontal flows concurrent with an X2.2 flare in the active region NOAA 11158

Horizontal proper motions were measured with local correlation tracking (LCT) techniques in active region NOAA 11158 on 2011 February 15 at a time when a major (X2.2) solar flare occurred. The measurements are based on continuum images and magnetograms of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The observed shear flows along the polarity inversion line were rather weak (a few 100 m s^–1). The counter‐streaming region shifted toward the north after the flare. A small circular area with flow speeds of up to 1.2 km s^–1 appeared after the flare near a region of rapid penumbral decay. The LCT signal in this region was provided by small‐scale photospheric brigthenings, which were associated with fast traveling moving magnetic features. Umbral strengthening and rapid penumbral decay was observed after the flare. Both phenomena were closely tied to kernels of white‐light flare emission. The white‐light flare only lasted for about 15 min and peaked 4 min earlier than the X‐ray flux. In comparison to other major flares, the X2.2 flare in active region NOAA 11158 only produced diminutive photospheric signatures (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solar activity forecast with a dynamo model

Although systematic measurements of the Sun's polar magnetic field exist only from mid-1970s, other proxies can be used to infer the polar field at earlier times. The observational data indicate a strong correlation between the polar field at a sunspot minimum and the strength of the next cycle, although the strength of the cycle is not correlated well with the polar field produced at its end. This suggests that the Babcock–Leighton mechanism of poloidal field generation from decaying sunspots involves randomness, whereas the other aspects of the dynamo process must be reasonably ordered and deterministic. Only if the magnetic diffusivity within the convection zone is assumed to be high (of order  10¹² cm² s⁻¹  ), we can explain the correlation between the polar field at a minimum and the next cycle. We give several independent arguments that the diffusivity must be of this order. In a dynamo model with diffusivity like this, the poloidal field generated at the mid-latitudes is advected toward the poles by the meridional circulation and simultaneously diffuses towards the tachocline, where the toroidal field for the next cycle is produced. To model actual solar cycles with a dynamo model having such high diffusivity, we have to feed the observational data of the poloidal field at the minimum into the theoretical model. We develop a method of doing this in a systematic way. Our model predicts that cycle 24 will be a very weak cycle. Hemispheric asymmetry of solar activity is also calculated with our model and compared with observational data.     

Calibration of Vector Magnetogram with the Nonlinear Least-squares Fitting Technique

To acquire Stokes profiles from observations of a simple sunspot with the Video Vector Magnetograph at Huairou Solar Observing Station (HSOS), we scanned the FeI λ5324.19A line over the wavelength interval from 150mA redward of the line center to 150 mA blueward, in steps of 10 mA. With the technique of analytic inversion of Stokes profiles via nonlinear least-squares, we present the calibration coefficients for the HSOS vector magnetic magnetogram. We obtained the theoretical calibration error with linear expressions derived from the Unno-Becker equation under weak-field approximation.     

Looking inside the butterfly diagram

The suitability of Maunder's butterfly diagram to give a realistic picture of the photospheric magnetic flux large scale distribution is discussed. The evolution of the sunspot zone in cycle 20 through 23 is described. To reduce the noise which covers any structure in the diagram, a smoothing algorithm has been applied to the sunspot data. This operation has eliminated any short period fluctuation, and given visibility to long duration phenomena. One of these phenomena is the fact that the equatorward drift of the spot zone center of mass results from the alternation of several prograde (namely, equatorward) segments with other stationary or poleward segments. The long duration of the stationary/retrograde phases as well as the similarities among the spot zone alternating paths in the cycles under examination prevent us from considering these features as meaningless fluctuations, randomly superimposed on the continuous equatorward migration. On the contrary, these features should be considered physically meaningful phenomena, requiring adequate explanations. Moreover, even the smoothed spotted area markedly oscillates. The compared examination of area and spot zone evolution allows us to infer details about the spotted area distribution inside the butterfly diagram. Links between the changing structure of the spot zone and the tachocline rotation rate oscillations are proposed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)